I would like to know the best procedure to completely remove a server from this ISPConfig multi server setup, knowing that all the data (let's say mail accounts and messages) has been transfered on a separate system like Zimbra for exemple.

So without bothering about the data, what should I do to have a clean ISPConfig multiserver setup with no mail server at all configured on it?

Is there some pitfall somewhere or I just have to modify the DNS MX entries, delete the domains under the Email Tab, delete the Mail server from System -> Server services and remove the Mail server's IP from the /etc/hosts file?